1. Field of the Invention
This invention relates to rpm detection systems for detecting the number of revolutions or rpm of automotive internal combustion engines and, more particularly, to an rpm detection system suited for the detection of the rpm of diesel engines.
2. Description of the Prior Art
The prior-art diesel engine rpm detection systems include one in which the engine rpm is detected through the detection of a pressure change at the time of fuel injection into the cylinder with a piezoelectric element or the like mounted on a fuel injection pump, and one in which the rpm detection is obtained through the detection of the reciprocating movement of the plunger of a metering pump provided within a fuel injection pump with a differential transformer.
However, the former system is greatly influenced by the fuel injection pressure in the diesel engine, and is prone to malfunction in a low engine rpm region in which the fuel injection pressure is low. Additionally, there is a fatal problem in that at the time when the fuel is cut, at which time no pressure change is produced in the pipe, the engine rpm cannot be detected.
Further, both the former and latter systems have a difficiency in that they can be applied only to diesel engines. Furthermore, with the latter system, sufficient performance cannot be obtained in the case where the system is installed in a separate place, for instance in an oil filler cap as according to the invention, unless its distance from the rocker arm is sufficiently small.
In an automotive engine, for instance a diesel engine as shown in FIGS. 1a and 1b which show the cylinder head in elevational and side sectional views, respectively, within an engine casing there is seen at least one valve rocker arm 13 when the interior of the engine casing is looked at through an oil inlet (a threaded hole) 11' provided in a cylinder head cover 11 by removing an oil filler cap 12 therefrom, the valve rocker arm 13 being oscillated according to the rotation of the engine crankshaft. This situation is found in almost all types of engines. In FIG. 1, designated at 14 is a cylinder head, and at 15 intake and exhaust valves which move vertically with the rocking motion of respective valve rocker arms 13.